1. Field of the Invention
The present invention relates to a ceramic dynamic pressure bearing, a motor with the bearing, a hard disk device, and a polygon scanner.
2. Description of the Related Art
Conventionally, a bearing for a motor which is a drive source of an electric appliance is in a form of a ball (or a xe2x80x9cball bearingxe2x80x9d).
Recently, a precision instrument such as peripheral device of computer has a motor that is rapidly increased in speed. For obtaining an excellent bearing capability (with reduction in: non-uniformity at low speed, abnormal noise, and vibration) and for keeping longevity, a dynamic pressure bearing is used. The dynamic pressure bearing is the one that uses fluid (such as air) as medium.
For example, the following dynamic pressure bearing is provided:
A main shaft and a bearing section (surrounding the main shaft) rotate around an axis. A fluid dynamic pressure is caused to a gap between an outer periphery of the main shaft and an inner periphery of the bearing section. The thus caused fluid dynamic pressure supports a rotation shaft.
Moreover, there is provided another bearing having a thrust surface (of the main shaft or the bearing section) that is supported with dynamic pressure.
At high speed with sufficient dynamic pressure, the dynamic pressure bearing is unlikely to cause contact between members facing each other across the dynamic pressure gap. Contrary to this, at low speed (such as starting and shutdown of rotation), the dynamic pressure bearing is likely to cause the contact between the members, due to insufficient dynamic pressure.
For component part of the above dynamic pressure bearing, a metal such as stainless metal, and a metal coated with resin and the like were generally used as material. The above metallic material are, however, likely to cause failures such as wear and seizure, attributable to the contact between the members at staring or shutdown. For preventing the wear and the seizure, a lubricant layer such as resin was applied to a section (of the member) facing the dynamic pressure gap, leaving insufficient effect.
For preventing the wear and the seizure securely, the members (the main shaft and the bearing) facing each other across the dynamic pressure gap are made of ceramic such as alumina.
The conventional dynamic pressure bearing with the dynamic pressure part made of the alumina ceramic is, however, not paid attention to in terms of material design, in other words, in respect of machining finish (accuracy/precision). Especially, the gap (between the outer periphery of the main shaft and the inner periphery of the bearing) causing a radial dynamic pressure is likely to cause a local wear attributable to low machining accuracy/precision of the outer periphery (of the main shaft) and the inner periphery (of the bearing section). Moreover, the low machining accuracy/precision of the outer periphery (of the main shaft) and the inner periphery (of the bearing section) may cause harmful effect on the dynamic pressure, losing uniformity and stability of rotation.
It is an object of the present invention to provide a ceramic dynamic pressure bearing that is unlikely to cause wear, seizure and the like at low speed at starting, shutdown and the like of rotation.
It is another object of the present invention to provide the ceramic dynamic pressure bearing that achieves a preferable rotation.
According to a first aspect of the present invention, there is provided a ceramic dynamic pressure bearing, comprising: a first member having a substantially cylindrical outer periphery which is formed with a gap forming surface for causing a radial dynamic pressure, and a second member having an inner periphery defining a substantially cylindrical through hole which is formed with a gap forming surface for causing the radial dynamic pressure. The first member is inserted into the through hole of the second member in such a manner as to form a radial gap between the gap forming surface of the first member and the gap forming surface of the second member. The first member and the second member make a rotation relative to each other so as to cause a fluid dynamic pressure at the radial gap. The first member is composed of an alumina ceramic comprising: an aluminum in a range from 90% to 99.5% by weight, where the figures in % are an Al2O3 conversion, and an oxide sintering assistant in a range from 0.5% to 10% by weight, where the figures in % are an oxide conversion. The outer periphery of the first member has a cylindricity not larger than 1.0 xcexcm, and a roundness not larger than 0.5 xcexcm which is measured in an arbitrary cross section perpendicular to an axis of the first member. The second member is composed of the alumina ceramic comprising: the aluminum in the range from 90% to 99.5% by weight, where the figures in % are the Al2O3 conversion, and the oxide sintering assistant in the range from 0.5% to 10% by weight, where the figures in % are the oxide conversion. The inner periphery defining the through hole of the second member has a cylindricity not larger than 1.5 xcexcm, and a roundness not larger than 1.0 xcexcm which is measured in an arbitrary cross section perpendicular to an axis of the second member.
According to a second aspect of the present invention, there is provided a motor comprising the ceramic dynamic pressure bearing as described above. The ceramic dynamic pressure bearing is used for bearing an output section of the motor.
According to a third aspect of the present invention, there is provided a hard disk device, comprising: a motor and a hard disk rotatably driven by the motor. The motor comprises the ceramic dynamic pressure bearing as described above.
According to a fourth aspect of the present invention, there is provided a polygon scanner, comprising: a motor and a polygon mirror rotatably driven by the motor. The motor comprises the ceramic dynamic pressure bearing as described above.
The roundness and the cylindricity are those specified, respectively, in item 3 and item 4 in xe2x80x9cAttached Tablexe2x80x9d in JIS B 0021 (1984), where JIS stands for Japanese Industrial Standard.
For securing accurate/precise measurement of the roundness and the cylindricity, the following measures may be taken:
The roundness and the cylindricity of the inner surface of the through hole of a second member are measured with a conventional profile measurement device for measuring profile of the inner surface. Hereinabove, the cylindricity is measured in cross sections (in required and sufficient number for securing accuracy/precision) perpendicular to the axis of the through hole of the second member.
Likewise, the roundness and the cylindricity of the outer surface of a first member may be measured with the conventional profile measurement device for measuring profile of the outer surface. Hereinabove, the cylindricity is measured in cross sections (in required and sufficient number for securing accuracy/precision) perpendicular to the axis of the first member.
If a hereinafter described dynamic pressure recess (groove) is to be formed, the roundness and the cylindricity are evaluated by excluding the area covering the dynamic pressure recess (groove).
According to the inventor of the present invention, the following points may be important in order to prevent the local wear from a radial dynamic pressure gap forming surface, and to secure uniform and stable dynamic pressure as well as rotation:
To keep the machining accuracy/precision of the radial dynamic pressure gap forming surface not lower than a predetermined level. More specifically, the cylindricity of the inner surface of the through hole of the second member are kept not larger than 1.5 xcexcm, while the roundness of the inner surface of the through hole of the second member in the arbitrary cross section perpendicular to the axis are kept not larger than 1.0 xcexcm. On the other hand, the cylindricity of the outer surface of the first member are kept not larger than 1.0 xcexcm, while the roundness of the first member in the arbitrary cross section perpendicular to the axis are kept not larger than 0.5 xcexcm.
Furthermore, the inventor of the present invention after further study found the following point may be effective for securing the machining accuracy/precision described above:
When the first member and the second member are made of alumina ceramic, alumina content is adjusted to 90% to 99.5% by weight.
Adjusting the alumina content in a range from 90% to 99.5% (or adjusting sintering assistant content) is attributable to the following cause:
If the sintering assistant is so increased excessively as to cause shortage of the alumina content, a liquid phase which may be caused during firing is increased. With the thus increased liquid phase, crystal grain of sintered body will grow excessively.
The above summarizes that the high content of the sintering assistant and the excessive growth of the crystal grain are responsible for lower hardness of ceramic organization. Therefore, the ceramic organization may make grinding resistance low, where the grinding resistance is required for finishing (using a grind stone or an abrasive grain) the dynamic pressure gap forming surface. With the low grinding resistance, the polishing speed is likely to become high unnecessarily, causing lower accuracy/precision of the polished surface. In other words, for improving the finish of the polished surface, the ceramic material has a proper hardness.
Therefore, securing the alumina content in the ceramic at least 90% by weight, or limiting the sintering assistant content not higher than 10% by weight may help prevent the above described excessive growth of the crystal grain.
Moreover, the xe2x80x9cat least 90%xe2x80x9d of the alumina content by weight or the xe2x80x9cnot higher than 10%xe2x80x9d of the sintering assistant by weight contributes to achieving the cylindricity (not larger than 1.5 xcexcm) of the inner surface of the through hole of the second member, and the roundness (not larger than 1.0 xcexcm) of the inner surface of the through hole of the second member in the arbitrary cross section perpendicular to the axis. On the other hand, the xe2x80x9cat least 90%xe2x80x9d of the alumina content by weight or the xe2x80x9cnot higher than 10%xe2x80x9d of the sintering assistant by weight also contributes to achieving the cylindricity (not larger than 1.0 xcexcm) of the outer surface of the first member, and the roundness (not larger than 0.5 xcexcm) of the first member in the arbitrary cross section perpendicular to the axis.
As a result, the local wear attributable to the machining accuracy/precision of the radial dynamic pressure gap forming surface is unlikely to occur during the operation of the dynamic pressure bearing. Moreover, the dynamic pressure at the radial dynamic pressure gap, and the rotation of the bearing become uniform and stable. Moreover, the alumina ceramic having a proper hardness improves resistance to wear which may be caused by contact between the members.
Contrary to the above, an excessive alumina content with reduced sintering assistant content may cause reduction of the liquid phase. With this, the crystal grain growth is controlled, to thereby lessen a mean crystal grain diameter to a great extent. As a result, the resistance to the polishing and the grinding may become too high, to thereby deteriorate machinability to a great extent.
With the aspects described above, the mean crystal grain diameter of the alumina ceramic is preferably in a range from 1 xcexcm to 7 xcexcm. The alumina content is preferably in a range from 92% to 98% by weight, more preferably from 93% to 97% by weight. Furthermore, the oxide sintering assistant constituting the grain boundary phase is preferably in a range from 2% to 8% by weight (oxide conversion), more preferably from 3% to 7% by weight (oxide conversion).
Dimension of the crystal grain (or surface vacancy) in this specification is defined as is seen in FIG. 6. More specifically, the organization of the dynamic pressure gap forming surface is viewed with an SEM (=scanning electron microscope), an optical microscope and the like. On a viewing area, a pair of parallel lines are drawn to an outline of the crystal grain (or surface vacancy) without crossing inside of the crystal grain (or surface vacancy). The pair of the parallel lines are drawn plural in number from different positions relative to the outline of the crystal grain (or surface vacancy). An arithmetic mean of a maximum dimension dmax and a minimum dimension dmin is defined as the dimension d of the crystal grain (or surface vacancy), namely, d=(dmax+dmin)/2.
As the oxide sintering assistant, an oxide having a cation selected from the group consisting of Li, Na, K, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, Si and the like is usable.
In this case, the at least one of the above cations is(are), in total, included in the alumina ceramic in a range from 0.5% to 10% by weight (preferably, 2% to 8% by weight; more preferably, 3% to 7% by weight), where the figures in % is based on oxide conversion.
Of the above cations, Si forms a bone of the grain boundary phase, to thereby increase strength. Moreover, Si improves fluidity of liquid phase.
The three alkaline metals Li, Na, and K decrease melting point of the liquid phase which is generated during firing. Thereby, Li, Na and K improve fluidity of the liquid phase, to thereby make the sintered body more compact (denser). Of the three alkaline metals, Na is low in cost. On top of that, although Na is, in principle, regarded as an impurity that is contained in ordinary alumina raw material powders such as those produced through Bayer process, Na is applicable to the sintering assistant. With a composition formula M2O, each of Li, Na and K is converted into oxide, where M is the cation metal element.
Following the three alkaline metals Li, Na, and K, the four alkaline earth metals Mg, Ca, Sr, and Ba feature an effect of decreasing the melting point of the liquid phase which is generated during firing. On the other hand, the four alkaline earth metals Mg, Ca, Sr, and Ba have an effect of strengthening the grain boundary phase when Mg, Ca, Sr, and Ba are absorbed in the grain boundary phase. As a result, Mg, Ca, Sr, and Ba improve strength and wear resistance of the entire sintered body and the dynamic pressure gap forming surface. Of the four alkaline earth metals, Ca shows the greatest effect. With a composition formula MO, each of the four alkaline earth metals Mg, Ca, Sr, and Ba is converted into oxide, where M is the cation metal element.
The rare earth metals Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu have advantages such as accelerating crystallization of the grain boundary phase, and increasing strength of the grain boundary phase. As a result, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, and Lu improve strength and wear resistance of the entire sintered body and the dynamic pressure gap forming surface. Of the rare earth metals, Ce shows the greatest effect. With a composition formula M2O3, each of the rare earth metals is converted into oxide, where M is the cation metal element.
Preferably, the alumina ceramic has an apparent density 3.5 g/cm3 to 3.9 g/cm3 (comparatively high). The thus highly controlled apparent density contributes to improvement in absolute values of strength and wear resistance of the alumina ceramic which constitutes the dynamic pressure gap forming surface. Moreover, the highly controlled apparent density effectively prevents wear from the dynamic pressure gap forming surface, which wear may be caused at starting and shutdown of rotation when members are likely to contact each other.
The alumina ceramic with an ideal compaction has a maximum density 4.0 g/cm3. Such a perfect compaction of the alumina ceramic, however, involves high sintering temperature, to thereby cause the crystal grain growth (unavoidable). With this, as the case may be, securing the accuracy/precision {1. Cylindricity (not larger than 1.5 xcexcm) of the inner surface of the through hole of the second member. 2 Roundness (not larger than 1.0 xcexcm) of the inner surface of the through hole of the second member in the arbitrary cross section perpendicular to the axis. 3. Cylindricity (not larger than 1.0 xcexcm) of the outer surface of the first member. 4. Roundness (not larger than 0.5 xcexcm) of the first member in the arbitrary cross section perpendicular to the axis.} of the radial dynamic pressure gap forming surface is of difficulty.
The alumina ceramic having the apparent density not higher than 3.9 g/cm3 does not involve high sintering temperature. Therefore, the crystal grain growth is controlled, to thereby secure the accuracy/precision (see former paragraph) of the radial dynamic pressure gap forming surface with convenience.
On the contrary, the alumina ceramic having the apparent density lower than 3.5 g/cm3 decreases the strength and the wear resistance of the alumina ceramic. Thereby, as the case may be, the wear is rather likely to occur to the dynamic pressure gap forming surface at starting and shutdown of rotation.
More preferably, the alumina ceramic has the apparent density in a range from 3.6 g/cm3 to 3.9 g/cm3.
Not only being sensitive to progress of the compaction, the apparent density of the alumina ceramic is more or less sensitive to types of the added sintering assistants as well as content of the added sintering assistant. When the correlation of ceramic compactness and crystal grain growth is at issue, a relative density is used. Herein, the relative density is a quotient which is obtained through a calculation dividing the apparent density by a true density, which true density is estimated from a composition ratio of the alumina and the sintering assistant. Under the present invention, the relative density of the alumina ceramic is not lower than 90%; more preferably, 90% to 98%; still more preferably, 94% to 97%.
The alumina ceramic whose density is adjusted as described above has the following features: 1. A relatively high flexural strength 280 MPa to 550 MPa. 2. Rockwell hardness 92 to 98 measured with load 15 N applied. 3. Fracture toughness 3 MPaxc2x7m1/2 to 5 MPaxc2x7m1/2. The alumina ceramic having the above flexural strength, Rockwell hardness and fracture toughness helps prevent and control wear (which may be caused on the dynamic pressure gap forming surface at starting and shutdown of rotation), and secures sufficient accuracy/precision of the radial dynamic pressure gap forming surface, without lowering excessively the machinability of polishing and grinding.
The flexural strength described in this specification means a 3-point flexural strength measured at room temperature based on a method specified in JIS R 1601 (1981).
The Rockwell hardness described in this specification is measured at room temperature based on a method specified in JIS Z 2245 (1992).
The fracture toughness described in this specification is measured based on an IF (=intermediate frequency) method specified in JIS R 1607 (1990).
The ceramic dynamic pressure bearing under the present invention has the second member in the direction of a rotation axis. At least one of two end surfaces of the second member faces a surface of a thrust plate. The end surface of the second member and the surface (facing the end surface of the second member) of the thrust plate constitute thrust dynamic pressure gap forming surfaces forming therebetween a thrust dynamic pressure gap.
Described hereinafter referring to FIG. 1 is a constitution of the ceramic dynamic pressure bearing.
The radial direction is perpendicular to the direction (upward and downward in FIG. 1) of the rotation axis of a main shaft. In FIG. 1, the outer periphery of the main shaft (first member) and the inner periphery of the bearing section (second member) are the radial dynamic pressure gap forming surfaces. The main shaft is fixed, while the bearing section is a rotor in a form of a cylinder.
If the ceramic dynamic pressure bearing is long in the direction of the rotation axis, whether a sufficient radial dynamic pressure is caused or not is of importance for stabilizing the rotation axis. The thrust direction is along the axis of the main shaft, in other words, the rotation direction (upward and downward in FIG. 1). In FIG. 1, the end surface of the bearing section and the surface (of the thrust plate) facing the end surface of the bearing section form the thrust dynamic pressure gap forming surfaces. The thrust dynamic pressure gap forming surface is allowed to have a slight inclination relative to a surface perpendicular to the rotation axis.
When the ceramic dynamic pressure bearing is short in the direction of the rotation axis, whether a sufficient thrust dynamic pressure is caused or not is of importance for stabilizing the rotation axis.
As is seen in FIG. 10, there is provided a bearing 251 having a main shaft 212 which is a rotor, and a cylindrical bearing section 221 which is fixed. The bearing 251 having the above constitution is allowed.
Hereinafter, as the case may be, the radial dynamic pressure gap and the thrust dynamic pressure gap are in combination referred to as xe2x80x9cdynamic pressure gap,xe2x80x9d and likewise, the radial dynamic pressure gap forming surface and the thrust dynamic pressure gap forming surface are in combination referred to as xe2x80x9cdynamic pressure gap forming surface.xe2x80x9d
Under the present invention, the alumina ceramic has a mean crystal grain diameter preferably in a range from 1 xcexcm to 7 xcexcm. More specifically, when the alumina ceramic is used for a material of the dynamic pressure bearing, surface condition of the ceramic dynamic pressure gap forming surfaces (of the main shaft and the bearing section) is of importance. In other words, the ceramic surface after polishing generally has minor vacancies which are supposed to cause a great effect to the rotation of the dynamic pressure bearing.
On the other hand, the dynamic pressure gap forming surface having an excessive smoothness, as the case may be, does not cause a sufficient fluid dynamic pressure to the dynamic pressure gap, according to study by the inventor of the present invention. When the dynamic pressure level is not sufficient, the rotation axis is not stable. With this, securing a stable rotation of the dynamic pressure bearing becomes difficult. Therefore, in order to keep the fluid dynamic pressure at high level, forming surface vacancies (having predetermined dimensions) aggressively on the dynamic pressure gap forming surface is effective.
The dynamic pressure gap forming surface is coated with a film which is thinner than a mean dimension of the surface vacancies. Preferably, the film is an amorphous carbon which is mainly made of hard carbon. Described below is a reason therefor:
Even if the dynamic pressure gap forming surfaces contact each other at low rotation (such as starting and shutdown) when the dynamic pressure is likely to get low, the amorphous hard carbon film helps prevent wear and adhesion.
Herein, the hard carbon (made of amorphous carbon) film has a main body which constitutes an amorphous skeleton, and has Vickers hardness not lower than 1,500 kg/mm2. The hardness of the film is measured with a dynamic ultra-minor hardness tester and the like (for example, NHT produced by CSEM in Switzerland).
Designing the film thinner than the mean dimension of the surface vacancies is for the following cause:
For preventing the surface vacancies from being excessively closed, which surface vacancies are formed aggressively for keeping the fluid dynamic pressure at high level, as described above.
Described below is in terms of the xe2x80x9cpredetermined dimensionsxe2x80x9d of the surface vacancies:
A method of forming the hard carbon film disclosed in Japanese Patent Examined Publication No. Heisei 6(1994)-060404 (equivalent of JP62116767 and JP1940883C) is usable. For depositing the hard carbon film without excessively closing the surface vacancies (that contribute to keeping fluid dynamic pressure at high level), however, the following method is more effective:
Allowing material steam (to be accumulated) to flow in such a manner as to form an inclined incident angle relative to a surface of a member.
More specifically, the ceramic dynamic pressure gap forming surface formed with large surface vacancies may disorder a fluid layer between the main shaft and the bearing. Thereby, a vibration may occur, for example, to the main shaft.
On the other hand, the ceramic dynamic pressure gap forming surface formed with small surface vacancies may cause an adhesion on the dynamic pressure gap forming surface of each of the main shaft and the bearing. Turning the bearing with the adhesion may cause the wear (hereinafter referred to as xe2x80x9cadhesion wearxe2x80x9d) and the like.
Furthermore, the ceramic dynamic pressure gap forming surface formed with extremely small vacancies does not contribute to causing the dynamic pressure.
The surface vacancies on the dynamic pressure gap forming surface are mainly formed by grain drop during polishing. Therefore, dimension (diameter) and distribution of the alumina ceramic crystal grain on the dynamic pressure gap forming surface are factors for forming the preferable surface vacancies, without failures described above. Under the present invention, controlling the mean diameter of the ceramic crystal grains (constituting the dynamic pressure gap forming surface of the member) in a range from 1 xcexcm to 7 xcexcm may contribute to an advantageous dimension and amount of the surface vacancies, in view of stability of the fluid dynamic pressure at high level and effective prevention of failures such as adhesion wear and linking at starting and shutdown of the dynamic pressure bearing.
The ceramic crystal grains having the mean diameter smaller than 1 xcexcm form the surface vacancies too small in mean diameter. Thereby, the adhesion wear and the linking are likely to occur at starting and shutdown of rotation of the dynamic pressure bearing. Moreover, with the ceramic crystal grains smaller than 1 xcexcm in mean diameter, the fluid dynamic pressure level is likely to become low, to thereby cause rotational deflection and the like. Hereinabove, the rotational deflection is defined as a maximum amplitude of a measuring point, and is perpendicular to the rotation axis.
On the other hand, the ceramic crystal grains having mean diameter larger than 7 xcexcm form the surface vacancies too large in mean diameter. Thereby, an excessive turbulent flow may occur at the dynamic pressure gap, to thereby cause vibration of the rotation axis.
The mean diameter of the ceramic crystal grains is preferably 2 xcexcm to 5 xcexcm.
On the dynamic pressure gap forming surface, a percentage area of the ceramic crystal grains having the grain diameter 2 xcexcm to 5 xcexcm not smaller than 40% (including 100%) is preferable for achieving the advantageous dimension and amount of the surface vacancies.
When the percentage area of the ceramic crystal grains having grain diameter 2 xcexcm to 5 xcexcm is smaller than 40% (in other words, when percentage area of the ceramic crystal grains having grain diameter larger than 5 xcexcm become great), the ceramic crystal grains are less likely to drop. Thereby, as the case may be, the percentage area of the surface vacancies that effectively contribute to causing the dynamic pressure becomes too small. On the other hand, when percentage area of the ceramic crystal grains having grain diameter smaller than 2 xcexcm become great, the formed surface vacancies are likely to become small in mean diameter. Both the above two cases, as the case may be, are disadvantageous for causing sufficient dynamic pressure.
The alumina ceramic is produced by firing a raw material which is a blending of alumina powders and sintering assistant powders. As is seen in FIG. 11, the thus produced alumina ceramic has an organization in which crystal grain of an alumina main phase (main element: alumina) is coupled with a grain boundary phase which is derived from the sintering assistant. Presumably, the crystal grain drop during polishing is mainly caused by fracture of the grain boundary phase. A space which was occupied by the crystal grain thus dropped remains as the vacancy that is open on the dynamic pressure gap forming surface.
The crystal grain drop is likely to occur especially in the following cases and the like where the strength of the grain boundary phase is low and coupling between the grain boundary phases is relatively decreased:
1. The grain boundary phase has a portion that is locally thin.
2. The grain boundary phase is insufficient due to an inner cavity and the like.
3. Cracks are caused by element segregation, thermal stress and the like.
In the specification, the term xe2x80x9cmain component (likewise, main, mainly and the like)xe2x80x9d is defined as a material having a content not smaller than 50% by weight, unless otherwise defined.
For example, as is seen in FIG. 12(a), a single crystal grain drops, to thereby form a single vacancy V1 having configuration and dimension corresponding to those of the single crystal grain thus dropped. Moreover, a plurality of crystal grains drops, to thereby form a group vacancy V2. Hereinabove in FIG. 12(a), white (void) areas indicate crystal grains that remain xe2x80x98not dropped,xe2x80x99 while black (dark) areas indicate crystal grains xe2x80x98dropped.xe2x80x99
FIG. 12(b) shows an ordinary condition where crystal grains having various dimensions are mixed in the organization. When a crystal grain having a large dimension is surrounded by a plurality of crystal grains having smaller dimensions, the surrounding smaller crystal grains drop with a chain reaction. Thereby, the surrounded large crystal grain drops, as the case may be. In this case, the vacancy thus formed has a dimension larger than those of individual crystal grains.
In the following situation, the group vacancy V2 (plurality of vacancies) is more likely to occur than the single vacancy V1 when a polishing force from the grind stone and the abrasive grain is applied to across a plurality of the crystal grains:
The alumina ceramic has an organization in which the individual crystal grain is equiaxial (namely, small anisotropy in configuration) and in which a portion having the decreased grain boundary phase coupling is spread to a predetermined extent.
In this case, the surface vacancies become larger in mean diameter than the crystal grains (mean diameter 1 xcexcm to 7 xcexcm).
Furthermore, the surface vacancies are so formed as to scatter substantially in the same direction (isotropic) on the dynamic pressure gap forming surface, irrespective of the polishing direction. With the surface vacancies larger than the crystal grains in mean diameter, the dynamic pressure level is further improved, to thereby achieve more stabilized rotation of the bearing.
When the alumina ceramic is produced, the mean grain diameter of the alumina powders used for the raw material is preferably in a range from 1 xcexcm to 5 xcexcm. In the case that the alumina powder having the mean diameter out of 1 xcexcm to 5 xcexcm are used, the mean diameter of the crystal grains of the sintered body obtained is not in the preferred range 1 xcexcm to 7 xcexcm (2 xcexcm to 5 xcexcm), as the case may be. A laser diffraction grain meter is used for measuring the mean grain diameter of the alumina powders.
The firing temperature is preferably in a range from 1,400xc2x0 C. to 1,700xc2x0 C. The firing temperature lower than 1,400xc2x0 C. is likely to decelerate compaction of the sintered body, to thereby reduce strength and wear resistance. On the other hand, the firing temperature higher than 1,700xc2x0 C. causes excessive crystal grain growth. Thereby, controlling the mean grain diameter of the crystal grains of the sintered body in a range from 1 xcexcm to 7 xcexcm (2 xcexcm to 5 xcexcm) is, as the case may be, of difficulty. Moreover, the firing temperature higher than 1,700xc2x0 C., as the case may be, causes deformation and the like of the sintered body, to thereby deteriorate dimension accuracy/precision.
The mean dimension of the surface vacancies on the dynamic pressure gap forming surface made of the ceramic is specifically in a range from 2 xcexcm to 20 xcexcm. Obtaining aggressively the mean dimension from 2 xcexcm to 20 xcexcm highly stabilizes the fluid dynamic pressure level. Moreover, in the case of the after described dynamic pressure bearing formed with the thrust dynamic pressure gap, the linking is prevented.
The mean dimension of the surface vacancies larger than 20 xcexcm is likely to cause an excessive turbulent flow to the dynamic pressure gap, to thereby vibrate the rotation axis.
On the other hand, the mean dimension of the surface vacancies smaller than 2 xcexcm is likely to cause the adhesion wear and the linking to the dynamic pressure gap forming surface at starting and shutdown of rotation. Moreover, the fluid dynamic pressure level at the dynamic pressure gap is likely to become low, to thereby cause rotational deflection (a maximum amplitude of a measuring point, and is perpendicular to the rotation axis) and the like. More preferably, the mean dimension of the surface vacancies is in a range from 5 xcexcm to 15 xcexcm.
The individual surface vacancies smaller than 2 xcexcm do not contribute so much to causing the dynamic pressure. On the other hand, so many surface vacancies larger than 20 xcexcm are likely to cause vibration and the like. The above two sentences summarize that the preferable dimension of the surface vacancy is from 2 xcexcm to 20 xcexcm for effective contribution to causing the dynamic pressure and to the stabilized rotation.
The percentage area of the surface vacancies from 2 xcexcm to 20 xcexcm on the dynamic pressure gap forming surface is not lower than 15%, and preferably, not lower than 20%, in view of the following aspects and the like:
1. Prevent seizure and linking from the dynamic pressure gap forming surface at starting and shutdown of rotation.
2. Increase fluid dynamic pressure level caused to the dynamic pressure gap.
On the other hand, the percentage area of the surface vacancies from 2 xcexcm to 20 xcexcm on the dynamic pressure gap forming surface is not higher than 60%, preferably not higher than 40%, in view of controlling more effectively the vibration and the like.
For effectively contributing to causing the dynamic pressure and for stabilizing the rotation, the surface vacancies have dimension in a range from 2 xcexcm to 20 xcexcm, and have the percentage area in a range from 10% to 60%.
In the specification, the percentage area of the surface vacancies is defined as a total area of the surface vacancies (observed on the dynamic pressure gap forming surface) divided by the area of the dynamic pressure gap forming surface. When a conventional dynamic pressure recess (groove) is formed on the dynamic pressure gap forming surface, however, an area covering the dynamic pressure recess (groove) is subtracted from the area of the dynamic pressure gap forming surface in the above calculation.
Measurement of the percentage area is carried out, for example, in the following manner:
1. Observe the dynamic pressure gap forming surface with a magnifying glass such as an optical microscope and the like.
2. Define a measurement area in a form of a square 300 xcexcmxc3x97300 xcexcm within an observatory view.
3. Divide the total area of the surface vacancies identified in the measurement area, by the measurement area.
For improving measurement accuracy/precision, the following steps are taken:
1. Arbitrarily define not less than five measurement areas in a single dynamic pressure gap forming surface, so as to obtain the percentage area of the surface vacancies.
2. Calculate a mean percentage area of the surface vacancies in the not less than five measurement areas.
On the dynamic pressure gap forming surface, the surface vacancy larger than 20 xcexcm is preferably prevented that may cause vibration and the like. More specifically, the percentage area of the surface vacancies larger than 20 xcexcm is preferably in not higher than 10%, more preferably, not higher than 5%. For preventing vibration, the maximum dimension of the surface vacancy on the dynamic pressure gap forming surface is preferably not larger than 100 xcexcm. In other words, the surface vacancy larger than 100 xcexcm is preferably not formed on the dynamic pressure gap forming surface.
Each of the first member and the second member forming the dynamic pressure gap is entirely made of the alumina ceramic. The alumina ceramic has an inner organization which is the compact sintered body with a small number of vacancies, and an outer organization which is the dynamic pressure gap forming surface formed with relatively a large number of vacancies. The above inner organization and outer organization of the alumina ceramic are preferable for increasing the dynamic pressure level, for preventing the adhesion wear and linking, and for improving the strength and wear resistance.
More specifically, the following condition is preferable:
In the alumina ceramic sintered body, the vacancies (having dimension from 2 xcexcm to 20 xcexcm) are locally formed as the xe2x80x98surface vacanciesxe2x80x99 on the dynamic pressure gap forming surface.
For efficiently forming the above organization, an effective measure is to form the surface vacancies by dropping the alumina ceramic crystal grains when finishing the dynamic pressure gap forming surface, as described above.
In the specification, the dynamic pressure bearing is allowed to be longer in the axial direction than an outer diameter of the thrust dynamic pressure gap forming surface. Otherwise, omission of the thrust dynamic pressure gap is allowed. With the constitution described in the above two sentences, the inclination of the rotor is controlled by the dynamic pressure caused to the radial dynamic pressure gap. More specifically, as is seen in FIG. 7, for example, there is provided a ceramic dynamic pressure bearing 33 having a long rotation shaft. When a bearing section 35 (rotor) is inclined, a pressure caused to a radial gap 38 regulates and rectifies the inclination of the bearing section 35.
On the other hand, in the specification, the dynamic pressure bearing is allowed to be shorter in the axial direction than the outer diameter of the thrust dynamic pressure gap forming surface. The inclination of the rotor during rotation is controlled mainly by the dynamic pressure at the thrust dynamic pressure gap. As is seen in FIG. 3, for example, there is provided a ceramic dynamic pressure bearing 3 having a short rotation shaft. When a bearing section 15 (rotor) is inclined, a pressure caused to a thrust gap 18A and a thrust gap 18B regulates and rectifies the inclination of the bearing section 35.
The dynamic pressure gap forming surface is allowed to be formed with a dynamic pressure recess (groove). For example, forming a conventional dynamic pressure recess (groove) on an outer periphery (the radial dynamic pressure gap forming surface) of the rotation shaft brings about smoother rotation.
As is seen in FIG. 2(a), there are formed a plurality of dynamic pressure recesses (grooves) which are disposed at predetermined intervals circumferentially on the outer periphery (radial dynamic pressure gap forming surface) of the shaft inserted into the bearing section. In FIG. 2(a), the recesses (grooves) are straight lines with an inclination defining a predetermined angle relative to a generating line of the outer periphery of the shaft. Other conventional patterns such as what is called herring bone are also allowed for the recesses (grooves), where the herring bone pattern has a circumferential base line at which ends of the recesses (grooves) are positioned.
Moreover, as is seen in FIG. 2(b), for example, a dynamic pressure recess (groove) is allowed to be formed on a surface (thrust dynamic pressure gap forming surface) of a thrust plate. In FIG. 2(b), a plurality of curved recesses (grooves) are formed circumferentially around a center of the thrust plate at predetermined intervals in such a manner that a distance from the center of the thrust plate is reduced gradually.
In the specification, the dynamic pressure bearing is effectively used for applications including, for example, the following accuracy/precision instruments:
1. A rotational main shaft section of a hard disk of a hard disk device.
2. A rotational main shaft section of a disk of a computer peripheral device including: CD-ROM (compact disk read only memory) drive, MO (magneto optical) drive, DVD (digital versatile disk) drive and the like.
3. A bearing for a rotational main shaft of a polygon mirror of a polygon scanner which is used for a laser printer, a copier and the like.
In the above precision instruments, a high speed not slower than 8,000 rpm is required for the bearing of the rotation drive. If more accuracy/precision is a concern, a still higher speed not slower than 10,000 rpm or not slower than 30,000 rpm is required.
Therefore, with an application of the ceramic dynamic pressure bearing in the specification, the fluid dynamic level is highly stabilized, and reduction in vibration and the like is achieved.
Moreover, in the specification, the following articles and the like are provided:
1. A motor which is equipped with the ceramic dynamic pressure bearing for bearing an output section of the motor.
2. A hard disk device which is equipped with the following articles:
1) The motor equipped with the ceramic dynamic pressure bearing.
2) A hard disk rotatably driven by the above bearing-equipped motor.
3. A polygon scanner which is equipped with the following articles:
1) The motor equipped with the ceramic dynamic pressure bearing.
2) A polygon mirror driven by the above bearing-equipped motor.
The other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.